MSDN Magazine - January 2008 - (Page 17)

Marshaling between Managed and Unmanaged Code YI ZHANG AND XIAOYING GUO L et’s face it. The world is not perfect. Very few com panies are developing entirely in managed code, and on top of that there’s a lot of legacy unmanaged code out there you need to work with. How do you integrate your managed and unmanaged projects? Does it take the form of calling into unmanaged code from a managed appli cation or calling into managed code from an unman Figure 1 Bridging the Gap between Managed and Unmanaged Code aged application? Fortunately, Microsoft® .NET Framework interop opens a chan and [OutAttribute] can be used: [InAttribute] only, [OutAttribute] nel between managed and unmanaged code, and marshaling plays only, [InAttribute, OutAttribute], and neither. When neither attri a very important role in that connection in that it allows for data bute is specified, you are telling the CLR to figure out directional exchange between the two (see Figure 1). There are many factors attributes by itself, with [InAttribute] usually applied by default. that affect the way the CLR marshals data between the unmanaged However, in the case of the StringBuilder class, both [InAttribute] and managed worlds, including attributes such as [MarshalAs], and [OutAttribute] are applied when neither attribute is specified. [StructLayout], [InAttribute], and [OutAttribute] as well as lan (See the later section on StringBuilder for details.) Also, the use of guage keywords such as out and ref in C#. out and ref keywords in C# could change the attributes that are ap Because of the sheer number of these factors, it can be a chal plied, as shown in Figure 2. Note that if no keyword is specified for lenge to marshal correctly, which requires you to understand many a parameter, it means that it is an input parameter by default. details in both unmanaged and managed code. In this column, Consider the code in Figure 3. There are three native C++ func we’ll cover the basic yet confusing topics you’ll encounter in your tions, and they are all making the same change to arg. Also, note everyday marshaling attempts. We won’t cover custom marshal ing, marshaling complex structures, or other advanced topics, but Figure 2 Out and Ref and Their Associated Attributes with a solid understanding of the basics, you’ll be ready to begin C# Keyword Attribute tackling those as well. [InAttribute] and [OutAttribute] (none specified) out ref [InAttribute] [OutAttribute] [InAttribute], [OutAttribute] The first marshaling topic we’ll discuss concerns the use of InAttribute and OutAttribute, attribute types located in the Sys tem.Runtime.InteropServices namespace. (When applying these attributes in your code, C# and Visual Basic® allow you to use the abbreviated forms [In] and [Out], but we will stick to the full names to avoid confusion.) When applying to method parameters and return values, these attributes control marshaling direction, so they’re known as direc tional attributes. [InAttribute] tells the CLR to marshal data from the caller to the callee at the beginning of the call, while [OutAttribute] tells the CLR to marshal back from the callee to the caller upon return. Both the caller and the callee can be either unmanaged or managed code. For example, in a P/Invoke call, managed code is calling unmanaged code. However, in a reverse P/Invoke, unman aged code could call managed code through a function pointer. There are four possible combinations in which [InAttribute] Figure 3 Trying Out Directional Attributes MARSHALLIB_API void __stdcall Func_In_Attribute(char *arg) { printf(“Inside Func_In_Attribute: arg = %s\n”, arg); strcpy(arg, “New”); } MARSHALLIB_API void __stdcall Func_Out_Attribute(char *arg) { printf(“Inside Func_Out_Attribute: arg = %s\n”, arg); strcpy(arg, “New”); } MARSHALLIB_API void __stdcall Func_InOut_Attribute(char *arg) { printf(“Inside Func_InOut_Attribute: arg = %s\n”, arg); strcpy(arg, “New”); } january2008 17

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2008

MSDN Magazine - January 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Cutting Edge IIS 7.0 - Enhance Your Apps with the Integrated ASP.NET Pipeline World Ready - Around the World with ASP.NET AJAX Applications WCF Syndication - HTTP Programming with WCF and the .NET Framework 3.5 Wicked Code Foundations Extreme ASP.NET {End Bracket}

MSDN Magazine - January 2008

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